Calculate age given the birth date in the format YYYYMMDD

javascript

prompt-engineering

functions

date-parsing

byAnton Shumikhin·Oct 26, 2024

Compute age in years from a YYYYMMDD formatted birth date with these steps:

Parse year, month, day from the input string.
Compare today's date with the birth date.
Subtract years, then adjust if the birth month/day has not occurred this year.

Example:

function ageFromBirthDate(birthDateString) {
  const now = new Date(); // Precious present moment
  // Extracts date parts and constructs birthDate object
  const birthDate = new Date(birthDateString.substr(0, 4), birthDateString.substr(4, 2) - 1, birthDateString.substr(6));
  
  const age = now.getFullYear() - birthDate.getFullYear(); // Simple subtraction, just like in grade-school math
  
  // If today is before the birthday, you're officially one year younger. Sorry!
  return (now.getMonth() > birthDate.getMonth() || 
          (now.getMonth() == birthDate.getMonth() && now.getDate() >= birthDate.getDate())) ? age : age - 1;
}

console.log(ageFromBirthDate("19900301")); // Your age...well depends on the year you're checking this!😉

Delving into the code logic

Here's a deep-dive into the key considerations regarding leap years and timezones:

Leap years

Leap years add an extra day to February. If you are one of the select few born on February 29, Date object's methods take care of this oddity.

The time play

The timezone of the user is crucial. When using Date.now(), your program should know it's thinking in local time, not GMT!

Milliseconds to years

getTime returns the time in milliseconds since the epoch. These need to be converted to years for a human-readable age. Nothing like telling someone they're about 1 billion seconds old to really make them feel their age!

Precision and edge case handling

Arguably the most important part of age calculation: precision and handling those pesky edge cases. Let's explore:

Precise age calculation

For a more precise age calculation that considers the time difference down to the milliseconds:

function preciseAge(birthDateString){
  // Construct birthDate object
  const birthDate = new Date(birthDateString.substr(0, 4), birthDateString.substr(4, 2) - 1, birthDateString.substr(6));
  
  // Conceptually similar to Marty McFly's DeLorean trip back to 1985
  const difference = Date.now() - birthDate.getTime();
  
  // Calculates the year difference, taking care of the strange epoch time (1970? Really?)
  return Math.abs(new Date(difference).getUTCFullYear() - 1970);
}

Date before birth date

What if the date of calculation is before the birth date? Edge case handling ensures your program doesn't time travel!

Leap day birthdays

Born on February 29? You might not get birthday cards every year, but your code will respect your real age, even on non-leap years.

Year change considerations

If born on December 31, the code ensures the age increments correctly after New Year's Eve. No missing out on that age tick!

Promoting flexibility

Your function becomes versatile with support for multiple birth date formats. Here's how to do it:

Creating a flexible date parser

function parseDate(birthDateString) {
  // Handles various date formats here and returns a Date object.
}

// Example Usage, supports a variety of date formats
const age = calculateAge(parseDate('01/03/1990')); // 'YYYYMMDD', 'MM/DD/YYYY', etc.

Consider using date parsing utilities or regular expressions to enhance flexibility further.

Enhancing with moment.js

When working with complex date-related calculations, libraries like moment.js simplify the process:

// Using moment.js for age calculation
function getAccurateAge(birthDateString) {
  const birthDate = moment(birthDateString, 'YYYYMMDD');
  const now = moment(); // Grabbing present moment in time
  return now.diff(birthDate, 'years'); // Diff, not just for Git anymore!
}